The present invention relates generally to systems and methods to detect disconnection of an indwelling vascular line, such as a catheter or needle, or its attached tubing. If not quickly detected, a disconnection can lead to rapid exsanguination, particularly when the blood in the catheter or tubing is under positive pressure. Examples of circumstances involving positive intravascular pressure include the positive pressure associated with an artery or arterio-venous fistula, or the positive pressure associated with an extracorporeal blood pump circuit. In hemodialysis, for example, a blood pump can generate blood flow rates of 400-500 ml/min, making rapid, reliable disconnect detection particularly desirable. Indeed any medical treatment involving relatively high flow or high pressure extracorporeal circulation (such as, for example, hemoperfusion or cardiopulmonary bypass) can be made safer by having an effective system to monitor the integrity of the arterial (withdrawal) and venous (return) blood lines.
In hemodialysis, for example, extracorporeal blood circulation can be accomplished with vascular access using either a single indwelling catheter, or two separate indwelling catheters. In a single catheter system, blood is alternately withdrawn from and returned to the body via the same cannula. A disconnection in this system can be quickly detected by placing an air monitor in the line at or near the pump inlet, because air will be drawn into the line from the disconnection site during the blood withdrawal phase of the pumping. On the other hand, in a two-catheter system, blood is typically continuously withdrawn from the body via one catheter inserted in a blood vessel or fistula, and returned to the body via the second catheter inserted in the same vessel some distance from the first catheter, or in a separate blood vessel altogether. In the two-catheter system, it is also possible to monitor for catheter or tubing dislodgement in the blood withdrawal or ‘arterial’ segment by using a sensor to detect the presence of air being entrained into the arterial tubing as blood is withdrawn from the blood vessel under negative pump pressure and/or positive fistula pressure. However, air-in-line detection cannot reliably detect a disconnection of the venous (return) segment of the extracorporeal circuit. In this case, if the blood-withdrawal path remains intact, air will not be introduced into the line. Thus it is particularly important to be able to detect a disruption in the continuity of the return line from the extracorporeal pump to the vascular access site.
Attempts have been made to develop systems to detect dislodgment based on the electrical, mechanical or acoustical properties of blood in the extracorporeal circuit. These systems have not been very effective because of the relatively high impedance of a blood circuit that includes long stretches of tubing, one or more blood pumps, valves, air traps and the like. Furthermore, the electrical interference generated by various devices along the blood path may obscure the signal that one is attempting to monitor.
An electrical signal can be introduced into the blood circuit through induction using a field coil surrounding a section of the blood tubing. It may also be introduced through capacitive coupling. For reasons of patient safety, the strength of an electrical signal introduced into the blood circuit necessarily must be small. However, the dielectric properties of the wall of the blood tubing can cause excessive noise or interference when attempting to detect conductivity changes in the blood from an electrical signal introduced through inductive or capacitive coupling. Therefore, it may be more desirable to introduce a brief, small electrical signal through direct contact with the blood path, to limit the length (and therefore impedance) of the blood path being monitored, and to perform the monitoring function at a suitable distance from any interference-producing components.